Electromagnetic microwaves, such as radar, could be used to determine filling levels in containers, and this method is since several years the dominating method for level gauging in containers tanks. The antenna is a critical part of such systems, and different types of antennas has been proposed and used, depending on e.g. the type of application, the filling material, the environmental conditions, the required measuring accuracy, etc.
One type of antenna that is widely used, and in many cases advantageous, is the flat array antenna An array antenna can e.g. create a narrow antenna beam without decreasing the measuring range by its vertical extension, which would be the case when using for instance a rod antenna, which is a common alternative for similar purposes. The antenna could be made with printed circuit board technique, and provides a flexible design capable of creating a variety of antenna functions. This makes it well suited for level gauging. This type of antenna could either be arranged inside the container, on a holding structure which extends into the container, or be arranged in level with the container wall, over an opening of the container. However, both said arrangement methods have severe problems in certain applications.
In U.S. Pat. No. 6,266,022 a flat array antenna mounted on a holding structure which extends into the container. When arranged on a holding structure entirely inside the container, it is difficult to achieve a tight sealing through the container wall, leading to severe leakage problems. Further, this arrangement requires free space in the top of the container, which is not always obtainable.
The arrangement of the antenna device directly in an opening of the container is also the subject of various problems. For example, it is difficult to achieve a tight sealing of the container opening. In fact, several separate leakage problems occurs simultaneously: leakage into the antenna device, which would affect the antenna functionality; leakage through the emitting surface of the antenna, which would affect and deteriorate the antenna electronics; and finally, leakage through the antenna device to the exterior.
Further, the temperature difference which normally prevails between the exterior and the interior of the container gives rise to condensation on the emitting surface of the antenna, which deteriorates the measuring performance of the antenna device. Another common problem in this type of arrangement is to obtain an adequate ability to withstand the high pressures which are often involved in this type of containers. Normal pressures could be in the range 16–40 bar.
A general problem affecting all flat array antenna devices is the problem of contaminations. For many applications the risk of contamination is a big issue. For level gauging in tanks used in e.g. food and pharmaceutical industry there is a mandatory need for internal surfaces which can be very efficiently cleaned. For example, even a few μm particles can easily hide bacteria capable of closing down a whole dairy for a few days. It can be noted that even a single or a few tenths of a mm of wet dirt can obstruct the function severely and that many times the combination of dirt and condensation, which is attracted by the dirt, is the worst case.
Another common requirement for antenna devices used for level gauging is to withstand a wide range of chemicals. This requirement is normally difficult to fulfill without using expensive materials, such as hastelloy, and the like.
Further, all prior art array antennas are inherently sensitive for leakage and corrosion as compared to horn antennas etc, since a packet of circuit boards is much more difficult to seal efficiently than to plug a circular waveguide (pipe).
It is therefore a need for a more effective flat array antenna, especially for mounting in a container wall opening, for use in level gauging.